Numerous reports in the literature indicate that changes occur in the mechanical behavior of hypertrophied cardiac myocardium as well as in the enzymatic properties of myosin isolated from it. Ongoing collaborative research in this laboratory has shown that the velocity of shortening, rate of force development tension-dependent heat production and myosin ATPase activity are reduced in cardiac hypertrophy secondary to pressure overload. The aim of the proposed research is to examine the hypothesis that there is a structural change in myosin from hypertrophied myocardium as an underlying cause of altered muscle performance. We will investigate two models of cardiac hypertrophy in the rabbit: 1) banding of the pulmonary artery which is known to result in cardiac myosin with depressed ATPase activity; and 2) thyroxine-treatment which leads to an elevated cardiac myosin ATPase activity. We will first investigate the hypothesis, based upon previous results from our laboratory, that the hypertrophic process causes a conformational change in the region of the reactive first class sulfhydryl (SH1) group of myosin heavy chains. The SH1 group of myosin from control and hypertrophied hearts will be labeled selectively with 14C-N-ethylmaleimide (14C-NEM), the myosins digested enzymatically, and the charge, amino acid composition and sequence of 14C-NEM peptides compared. We will also determine the phosphate content of myosin light chain 2 from normal and hypertrophied hearts based on recent evidence that the degree of phosphorylation of this light chain varies with the contractile state of the heart. Experiments will also include an analysis of the influence of phosphorylation on the interaction of actin and myosin from control and hypertrophied hearts. These studies will provide information regarding the changes which occur in myosin during the development of hypertrophy as well as the extent to which these changes might explain alterations known to occur in the function of hypertrophied myocardium. The studies will also contribute significantly to our understanding of the enzymology of myosin by elucidating the primary structure of a region of the molecule known to be involved in its enzymatic function.